Definitely "The Underdwellers".
It was one of the earliest episodes they produced, but they delayed its original air date because it was a weak episode and they knew it.
Conceptually it's fine, Batman discovering an underground system of orphans subjegated under a so-called "Sewer King" is a good scenario that adds to Bruce Wayne's character, but the execution of the episode was poor.
Character dialogue in particular was weird and stilted, and stuff like having an orphan in Bruce Wayne's manor, whilst being Batman in all parts of it, and snatching a gun that the orphan started playing with.
Bruce Wayne having rifles on display in his home doesn't make a lot of sense, it was part of some forced PSAs like "don't play with guns", and the opening non-sequiter of children literally playing on top of trains.